1. Field of the Invention
The present invention relates to an image formation apparatus and in particular, to an electron-photographic image formation apparatus using a liquid developing agent.
2. Description of the Related Art
In a conventional image formation apparatus performing development by supplying a developing liquid containing toner to a photosensitive body, development is performed by a developing solution supplied from a developer supply inlet into a clearance between a development roller and a belt-shaped photosensitive body. In this development, an necessary developing solution adhered to the belt-shaped photosensitive body is wiped away by a squeeze roller. The squeeze roller is rotatably supported by a squeeze support body which is mounted via a compression coil spring on a moving frame that can vertically move toward and away from the film-shaped photosensitive body. When the moving frame comes nearer to the film-shaped photosensitive body, the squeeze roller is pushed to into abutment with the film-shaped photosensitive body with a greater depression force of the elasticity of the compression coil spring, and when the moving frame goes apart from the film-shaped photosensitive body in a reach of the compression coil spring, the squeeze roller is in abutment with the film-shaped photosensitive body with a smaller depression force of the compression coil spring.
Normally, the moving frame is in the proximity of the belt-shaped photosensitive body, so that the squeeze roller is pushed against the belt-shaped photosensitive body with a strong force of the compression coil spring and is rotated in the same direction as the moving direction of the surface of the belt-shaped photosensitive body. This rotation performs the squeezing, i.e., wipe off of the unnecessary developing solution. As a result, the image on the belt-shaped photosensitive body is film-shaped, which is then subjected to a drying process before transferred onto a paper.
In this squeeze operation, the unnecessary developing solution squeezed from the belt-shaped photosensitive body by the squeeze roller remains at the upstream side to the contact portion between the belt-shaped photosensitive body and the squeeze roller. If the apparatus is stopped in this state, the remaining developing solution is dried into a solid state, which is sure to cause stains on the next printing.
For this, in this type of image formation apparatus, as disclosed in Japanese Patent Publication (unexamined) 5-273867, the squeeze roller is maintained at a certain distance from the photosensitive body and rotated in the opposite direction so as to remove an unnecessary portion of the developing solution without distorting the toner image being developed. More specifically, immediately before stopping operation of the image formation apparatus, the moving frame holding the squeeze support bodies at the both sides of the squeeze roller is moved away from the belt-shaped photosensitive body to expand the compression coil spring, thus reducing the pressing force of the squeeze roller to the belt-shaped photosensitive body. This enables the squeeze roller to rotate in the opposite direction to the movement direction of surface of the belt-shaped photosensitive body. Thus, by rotating the squeeze roller in the opposite direction, it is possible to effectively remove the remaining developing solution from the belt-shaped photosensitive body.
Moreover, Japanese Patent Publication (unexamined) 58-18666 discloses an invention for wiping down the remaining developing solution by using a blade from the surface of the squeeze roller. This is because if the developing solution remains on the squeeze roller, it is inconvenient for removing an unnecessary portion of the developing solution from the belt-shaped photosensitive body.
However, the conventional image formation apparatuses have various problems.
Firstly, in the conventional image formation apparatus, a single compression coil spring provided in the squeeze support bodies at both ends of the squeeze roller was the only means for changing the force pushing the squeeze roller to the belt-shaped photosensitive body so as to switch between the follower rotation (during a printing) and the opposite-direction rotation (during a cleaning).
However, the optimal pushing force of the squeeze roller for the opposite-direction rotation of the squeeze roller is as small as 1/10 of the optimal pushing force for the follower rotation. Moreover, the compression coil spring should have a short free length for reducing the size of the image formation apparatus. With such single compression coil spring having a short free length, it has been difficult to obtain optimal pushing forces for the follower rotation and the driven rotation.
That is, when it is possible to obtain an optimal pushing force for the follower rotation or the driven rotation, it is impossible to obtain an optimal pushing force for the other rotation. If an optimal pushing force cannot be obtained for the follower rotation (during a printing), an image on the belt-shaped photosensitive body after squeezing cannot be made into a film shape, causing a transfer failure or deterioration of the image. If an optimal pushing force cannot be obtained for the driven rotation (during a cleaning), the developing solution remaining between the squeeze roller and the belt-shaped photosensitive body cannot be removed completely or the belt-shaped photosensitive body may be scratched.
Moreover, as a second problem, in a conventional color image formation apparatus, it is necessary to provide four developers for the three primary colors of Yellow, Magenta, Cyan, and Black, which makes it difficult to reduce the size and weight of the image formation apparatus.
For example, the image formation apparatus requires a frame having a high rigidness for supporting a large load as well as a large-size motor for driving the apparatus. This also increases the production cost of the image formation apparatus.
Furthermore, as a third problem, if the squeeze roller having a liquid developing agent adhered thereto is left as it is for a long period of time, the developing solution on the squeeze roller is dried and toner is adhered to the roller surface, deteriorating its performance, causing an image by black stripes.
Even when the blade is brought into abutment with the squeeze roller, it has been difficult actually to completely remove the developing agent adhered to the squeeze roller. If the remaining developing agent is dried, it cannot be removed by the blade.
Thus, in the conventional image formation apparatus, the squeeze roller cannot have a service life as long as the service life of other components including the developing roller. It has been necessary to periodically clean the apparatus by a user or a maintenance staff.